The field of the invention is systems and methods for radiation treatment planning. More particularly, the invention relates to systems and methods for producing a treatment plan for an image-guided radiation treatment system that accounts for utilizing the imaging dose as part of the treatment dose.
Numerous studies over the years have demonstrated the feasibility of both marker and markerless based kiloVolt (“kV”) fluoroscopic tracking. Compared to external patient surface monitoring, using either IR markers or 3D surface imaging, kV tracking has the benefit of direct tumor position monitoring, and therefore avoids potential issues with poor external-to-internal tumor correlation.
Another advantage of kV imaging is its ability to resolve lower contrast anatomical information than megavolt (“MV”) electronic portal imaging devices (“EPID”), which typically rely on the use of surgically implanted fiducial markers for target location. The use of such markerless kV tracking is especially attractive for the lung, where percutaneous implantation of fiducial markers is an invasive and costly surgical procedure that carries the risk of pneumothorax. Although a combined MRI-LINAC approach is one potential modality for performing markerless real-time lung tumor motion tracking, such systems are still in the early developmental stage, and are likely to be extremely complex and expensive. Because most modern LINACs are now pre-equipped with on-board kV imaging sources, performing real-time lung tumor tracking through the use of kV imaging is a potential cost effective solution.
Clinical adoption of real-time kV based tracking has been held back by concern over the excess kV imaging dose cost to the patient when imaging in continuous fluoroscopic mode. This includes tracking with two or more kV imagers mounted either to the ceiling or the floor of the treatment room, or two kV imagers mounted to the gantry, or a single kV imager mounted on the LINAC gantry.
Because the problem of high imaging dose associated with real time kV fluoroscopic tumor tracking has long been acknowledged, there are a number of recent studies investigating techniques to reduce the kV imaging dose. Many of these studies use additional information to limit the frequency of kV imaging. In one such study, stereoscopic MV-kV imaging of a fiducial marker is performed at the start of therapy in order to build a correlation model, so that, subsequently, kV imaging is only required intermittently when the fiducial is not visible in the MV image.
Another study, reported on the Cyberknife Synchrony system. In this system, an internal/external correlation model was first developed in order to track the tumor motion via externally visible marker tracking. However, the model was updated throughout the course of treatment via the acquisition of new x-ray images once every 1-5 minutes. The study reported that the synchrony system reduced the 3D positional error, though some error was still present. Other studies have investigated coupling fluoroscopy with external surface cameras, rather than external marker cameras.
The dosimetric effect of real-time motion tracking with kV imaging and fiducial markers have been investigated, with numbers often in the range of 1 cGy per minute at the surface for relatively low mAs. Depending on the imaging requirements, the fluence rate may escalate, suggesting that for real-time kV planar imaging to be incorporated safely into image-guided radiation therapy IGRT procedures, the dosimetric effects should be carefully considered prior to treatment. This is especially a concern for markerless tracking, where low contrast tissues and large anatomical features may require the use of higher kV fluences and larger kV aperture sizes than with metallic fiducial markers. Due to the variety of patient anatomical sizes, and the complexity of the kV beam arrangements typically needed for real-time tracking, it is necessary to perform a patient specific kV dose calculation in order to fully understand the exact impact of the kV imager.
Volumetric measurements, such as cone beam CT (“CBCT”), have been shown to improve the accuracy of patient setup for lung stereotactic body radiotherapy (“SBRT”), conventional radiotherapy, and IMRT (Bissonnette, et al., 2009) (Den, et al, 2010). As a result, daily CBCT is increasingly included as a part of patient treatment protocols. Daily in-room CBCT can take one of two forms: Megavoltage (MV) CBCT (Pouliot, et al., 2004), or kilovoltage (kV) CBCT (Schreibmann, et al., 2005). Measurements of radiation dose for either technique have been previously studied (Hyer, et al., 2010) (Islam, et al., 2006) (Gayou, et al., 2007), and at this point, incorporation of MV CBCT dose into the treatment plan has been reported on (Miften, et al., 2007).
There have been early investigations into the incorporation of kV CBCT dose into the treatment plan (Alaei, et al., 2009) (Dzierma, et al., 2013). In one such study, in an anthropomorphic phantom, thoracic CBCT was found to result in doses in the range of 6.04-8.98 mGy in the lungs and 3.93-6.23 mGy in the spinal cord, among reported dose values at other typical organ at risk (OAR) sites. These values will tend to be specific to both patient geometry and imaging protocol.
CBCT and fluoroscopy share the similarity that one of the primary concerns of either imaging procedure is excessive skin dose. Therefore a method of incorporating kV CBCT dose into the patient treatment plan at the point of treatment planning, and assessing the effect of the kV CBCT dose on the overall treatment plan quality and on skin dose reduction is needed.